Giardia lamblia is a significant cause of diarrheal disease worldwide which is perpetuated by the infective cyst form of the parasite. The mechanisms underlying the process of differentiation from trophozoite to cyst are poorly understood. The overall purpose of this project is to decipher the biochemical and molecular events involved in this process, focusing on structure and function of carbohydrate moieties in both trophozoite and cyst. The specific aims of this proposal are directed at a) elucidating structural and biosynthetic aspects of N-acetyl-glucosamine containing carbohydrate moieties which have been identified as chitin in cyst walls as well as linked to glycoproteins in trophozoites and cysts and b) relating these aspects to the biological role of these moieties in differentiation of the parasite. The first aim is to determine the structure and linkage of the GIcNAc residues in trophozoite and cyst glycoproteins with the long term objective of studying their biosynthesis and ascertaining whether they serve as precursor moieties for chitin synthesis which is a key event in differentiation. The second aim is to characterize, purify and produce antibodies to chitin synthase which is a developmentally regulated enzyme involved in the biosynthesis of chitin. These studies will facilitate the long term objective of delineating genetic control of differentiation at the molecular level by identifying cyst and trophozoite specific genes encoding developmentally regulated proteins. The third aim is to determine the subcellular localization of chitin and chitin precursors as well as the N-acetyl-glucosamine containing glycoproteins at the ultrastructural level. The last aim is to relate the findings obtained at the structural and biochemical level to growth and differentiation of the parasite in vitro as well as in vivo in animal models of giardiasis. The ultimate goal is to design strategies to arrest differentiation, interrupt the life cycle of the parasite and control spread of the disease.